1. Field of the Invention
The present invention generally relates to power supplies and DC to DC converters and, more particularly, to multiphase power supplies and converters which operate at low output voltages and increased input to output voltage differentials.
2. Description of the Prior Art
Advances in microprocessor technologies impose challenges in designing power supplies. In particular, there has been a strong incentive to develop increased integration density since increases in performance in terms of signal propagation time, noise immunity, increased functionality per chip and the like can be achieved thereby as well as some improvements in manufacturing economy. However, to exploit the reduced signal propagation time, clock speeds have been increased; increasing dissipated power. As a consequence, to limit power dissipation, operating voltages have been decreased in modern designs and further reduction in operating voltage can be anticipated. At the same time, supply voltage tolerances have been decreased while potential transient power requirements have increased and will present even more stringent requirements in the future.
In order to deliver a highly accurate supply voltage to microprocessors, a dedicated DC/DC converter is usually placed in close physical and electrical proximity to the processor or other chip or module having such requirements. Such a converter is often provided in a modular form and is referred to as a voltage regulator module (VRM). Most common of the currently used VRM circuit topologies are multiphase (e.g. two or more phase) buck converters; the principal benefits of which are ripple voltage cancellation effect, efficiency, relatively small module size and the ability to use relatively inexpensive components. More specifically, the ripple cancellation effect between the phases allows use of small inductances to improve transient response and minimization of output filter capacitance. As is well-understood in the art, a ripple voltage is the voltage above the converter output voltage which drives input current to the output filter capacitor when the voltage is drawn down below the nominal converter output voltage by the load (represented in this case by a microprocessor or other chip or module).
Use of more than two phases can interleave the conductor currents between the individual phase channels and thereby greatly reduce the the total ripple currents flowing into the output capacitor(s). Such further reduction of ripple current by use of three or more phases allows use of even smaller inductors to improve transient response and allows a small capacitance to meet transient requirements. Reduced ripple voltage also allows more room for voltage deviations during transients since less of the voltage tolerance budget will be consumed by the ripple voltage.
As is well-understood, the total current that can be supplied by a DC/DC converter is a function of both the ripple voltage and its duty cycle. That is, in buck converter VRMs, such as the two phase buck converter shown in FIG. 1, the duty cycle, D, is the ratio of the output voltage V0 to the input voltage, VIN. In earlier VRMs, the input voltage and the output voltage both approximated 5 volts, in which case, the synchronous buck topology works very well to minimize the ripple voltage.
However, current microprocessors for desktop computers, workstations and and low-end servers require the VRMs to work with a 12 volt input while the microprocessor voltage will generally be reduced from the 5 volts required in earlier generations of microprocessors. In laptop computers, VRMs directly step down from the 16 to 24 volt battery charger voltage to a microprocessor voltage of 1.5 volts. It can be anticipated that future generations of microprocessors will operate at voltages well below 1 volt to further reduce power dissipation. Accordingly, the VRMs will be required to operate at very small duty cycles. It follows that the ripple voltage and current will thus be increased, even in converter topologies which provide a significant ripple cancellation effect. FIG. 2 shows the influence of duty cycle on the output current ripple for buck converters of different numbers of phases (normalized against the inductor current ripple at zero duty cycle. It can be seen that at small duty cycles where the current ripple reduction is poor, the benefits of increasing the number of phases of the buck converter is compromised, as well.
Further, study and simulation have shown that with very small duty cycles, both transient response and efficiency of multiphase buck converters suffer as well. FIG. 3 shows the measured efficiency comparison (including power losses in the power stage but excluding control and gate drive losses) between input voltages of 5 volts (D=0.3) and 12 volts (D=0.125). As can be seen, the 5 volt input VRM can achieve 87% efficiency at full load and 91% peak efficiency while the 12 volt input VRM can achieve only 81% efficiency at full load and 84.5% peak efficiency. In other words, the increase of duty cycle, D, from 0.125 to 0.3 increases full load efficiency by 6% and peak efficiency by 7%. The loss contributions indicate that the efficiency improvement at longer duty cycles is mainly caused by the reduced switching loss of the top switches (S1 and S3 of FIG. 1). The switching loss is proportional to the switching current and voltage stress. Therefore, higher differential input and output voltages result in higher switching losses and reduced efficiency.
In summary, while similarity of input and output voltages of VRMs and multiphase buck converters in particular in the past has yielded satisfactory performance and good efficiency, the current practical trends toward increased VRM input voltage and reduced output voltage compromises both of these principal qualities and can be expected to increase in the future. The problem and criticality of efficiency is also compounded since reduced efficiency implies increased power dissipation of the VRM in the physical proximity to the load/microprocessor (to limit ohmic voltage drop) increases the complexity and criticality of heat removal from both the VRM and the load. Perhaps more importantly, however, the increased ripple voltage/current at short duty cycles compromises transient response of the VRM.
It is therefore an object of the present invention to provide a VRM of increased efficiency without compromise of transient response.
It is another object of the invention to provide a voltage regulator module which can be fabricated at small size with relatively inexpensive component while achieving improved performance.
In order to accomplish these and other objects of the invention, a converter circuit and electronic device including a converter circuit and an integrated circuit powered thereby is provided in which the converter circuit comprises a plurality of branches switched in a complementary manner to define a plurality of phases, an inductor connected in series with an output of each said phase, and a clamping circuit connected to a tap of the inductor and/or an arrangement for providing coupling between respective inductors of respective neighboring ones of said plurality of phases.